Principles of Inheritance and Variation

The One Thing

Inheritance depends on the transmission and interaction of genes and alleles carried on chromosomes. Mendelian patterns explain many traits, but allele interactions, gene interactions, linkage, recombination, mutation, chromosome changes and environmental effects produce important deviations and sources of variation.

Reactions, Processes and Experiments

Key Terms

• Heredity: The transmission of biological characteristics from parents to offspring.
• Variation: Differences in characteristics among individuals of the same species, caused by genetic and environmental factors.
• Gene: A functional unit of heredity and a segment of DNA that influences a particular trait or produces a functional RNA or protein.
• Allele: An alternative form of a gene present at the same locus on homologous chromosomes.
• Locus: The specific position occupied by a gene on a chromosome.
• Chromosome: A DNA-protein structure that carries genes and becomes visibly condensed during cell division.
• Genotype: The genetic constitution of an organism, such as TT, Tt, or tt.
• Phenotype: The observable characteristics of an organism produced by its genotype and environmental effects.
• Homozygous: Having two identical alleles for a gene, such as AA or aa.
• Heterozygous: Having two different alleles for a gene, such as Aa.
• Dominant allele: An allele expressed in the phenotype of a heterozygote.
• Recessive allele: An allele expressed in the phenotype only when present in the homozygous condition, in simple complete dominance.
• Mendel's law of dominance: In a heterozygote, one allele may mask the expression of another allele for the same trait.
• Law of segregation: The two alleles of a gene separate during gamete formation, so each gamete receives only one allele.
• Law of independent assortment: Allele pairs of different genes assort independently during gamete formation when the genes are unlinked or sufficiently far apart.
• Monohybrid cross: A genetic cross that follows the inheritance of one pair of contrasting traits.
• Dihybrid cross: A genetic cross that follows the inheritance of two pairs of contrasting traits.
• Test cross: A cross between an individual showing a dominant phenotype and a homozygous recessive individual to determine the unknown genotype.
• Back cross: A cross between an offspring and either of its parents or a genetically similar parental individual.
• Incomplete dominance: A condition in which neither allele is completely dominant, producing an intermediate heterozygous phenotype.
• Codominance: A condition in which both alleles are fully and separately expressed in a heterozygote.
• Multiple alleles: The presence of more than two alternative alleles for a gene in a population, although an individual carries only two.
• Pleiotropy: The phenomenon in which one gene influences two or more apparently unrelated traits.
• Polygenic inheritance: Inheritance of a trait controlled by several genes, often producing continuous variation, as in human skin colour.
• Epistasis: Interaction in which one gene masks or modifies the expression of another gene at a different locus.
• Linkage: The tendency of genes located close together on the same chromosome to be inherited together.
• Crossing over: The exchange of corresponding DNA segments between non-sister chromatids of homologous chromosomes during pachytene of prophase I of meiosis.
• Recombination: The formation of new combinations of alleles, commonly produced by crossing over and independent assortment.
• Recombination frequency: The percentage of recombinant offspring and an estimate of the distance between linked genes.
• Sex-linked inheritance: Inheritance of genes located on sex chromosomes, commonly the X chromosome.
• Pedigree analysis: The study of a family tree using standard symbols to trace the inheritance of a trait or disorder.
• Mutation: A sudden, stable and heritable change in genetic material.
• Aneuploidy: A condition in which one or more chromosomes are gained or lost, usually because of nondisjunction.
• Down syndrome: A chromosomal disorder caused by trisomy of chromosome 21, commonly associated with characteristic facial features, developmental delay and congenital abnormalities.
• Klinefelter syndrome: A sex-chromosome disorder usually represented as 47,XXY, occurring in phenotypic males and often associated with small testes and infertility.
• Turner syndrome: A sex-chromosome disorder usually represented as 45,X, occurring in phenotypic females and often associated with short stature and infertility.
• Thalassemia: An autosomal recessive disorder involving reduced production of globin chains and resulting in anaemia.
• Sickle-cell anaemia: An autosomal recessive disorder caused by a mutation in the beta-globin gene, producing abnormal haemoglobin and sickle-shaped red blood cells under low-oxygen conditions.
• Phenylketonuria: An autosomal recessive metabolic disorder caused by inability to properly metabolise phenylalanine, which may lead to intellectual disability if untreated.
• Haemophilia: An X-linked recessive disorder in which blood clotting is impaired.
• Colour blindness: An X-linked recessive condition involving difficulty distinguishing certain colours, commonly red and green.
• Downstream genetic counselling: Professional guidance that explains genetic risks, testing options, inheritance patterns and reproductive choices.

Easily Confused

• Genotype and phenotype: Genotype is the genetic constitution; phenotype is the observable characteristic produced by genotype and environmental effects.
• Homozygous and heterozygous: Homozygous individuals have identical alleles; heterozygous individuals have different alleles.
• Dominant and recessive alleles: A dominant allele is expressed in a heterozygote; a recessive allele is expressed only in the homozygous condition in simple complete dominance.
• Dominance and allele frequency: Dominance describes expression in a heterozygote; it does not mean that an allele is more common, stronger or necessarily better.
• Incomplete dominance and codominance: Incomplete dominance produces an intermediate heterozygous phenotype; codominance produces simultaneous, separate expression of both alleles.
• Monohybrid and dihybrid crosses: A monohybrid cross follows one gene pair; a dihybrid cross follows two gene pairs.
• Test cross and back cross: A test cross uses a homozygous recessive individual to determine an unknown genotype; a back cross involves an offspring and either parent or a genetically similar parental individual.
• Linkage and independent assortment: Linked genes on the same chromosome tend to be inherited together; unlinked genes or genes sufficiently far apart assort independently.
• Crossing over and recombination: Crossing over is the physical exchange of DNA between non-sister chromatids; recombination is the resulting formation of new allele combinations.
• Mutation and aneuploidy: Mutation is a stable heritable change in genetic material; aneuploidy specifically involves gain or loss of one or more chromosomes.
• Autosomal recessive and autosomal dominant inheritance: Autosomal recessive disorders may occur in children of unaffected carrier parents; autosomal dominant disorders may appear in successive generations.
• X-linked inheritance and autosomal inheritance: X-linked traits involve sex chromosomes and often show sex-related transmission patterns; autosomal traits involve non-sex chromosomes and generally affect males and females similarly.

What Gets Asked

• Calculate monohybrid ratios: For Aa x Aa, give 1 AA : 2 Aa : 1 aa genotypically and 3 dominant : 1 recessive phenotypically under complete dominance. The common error is confusing genotypic and phenotypic ratios.
• Calculate dihybrid and test-cross ratios: For AaBb x AaBb, use 9 : 3 : 3 : 1 only when genes assort independently and show complete dominance. For AaBb x aabb, use 1 : 1 : 1 : 1 only under independent assortment.
• Apply probability rules: Use the product rule for simultaneous independent events and the sum rule for alternative mutually exclusive events. The relevant expression is P(event) = favourable outcomes / total possible outcomes.
• Identify non-Mendelian inheritance: Distinguish incomplete dominance, codominance, multiple alleles, pleiotropy, polygenic inheritance and epistasis. In particular, do not describe codominance as an intermediate phenotype.
• Interpret linkage and mapping data: Use Recombination frequency = number of recombinant offspring / total offspring x 100; 1% recombination frequency corresponds approximately to one map unit or one centimorgan. Lower recombination frequency indicates closer genes.
• Interpret human pedigrees and disorders: Recognise that X-linked recessive traits are more frequent in males and are not transmitted directly from fathers to sons; autosomal recessive disorders may arise from unaffected carrier parents, while autosomal dominant disorders may appear in successive generations.